Image suppression system



Jun# 9, 1944712-` F. N. JACOB 2,285,979 IMAGE `SUPPRESSION SYSTEM Filed May 15, 1941 NVENT OR.

Patented June 9, 1942" IMAGE SUPPRESSION SYSTEM Frederick N. Jacob. Chicago, Ill., assignor to Johnson Laboratories, Inc., Chicago, Ill., a corporation of Illinois Application May 15, 194i, serial No. 393,538

(ci. o-2o) 8 Claims.l

The present invention relates particularly to receiving systems of the superheterodyne type in which the signal passes through one or more resonant circuits that are tuned to the signal frequency and is mixed with locally generated oscillations to produce a predetermined beat frequency at which it is amplified and which is generally termed the intermediate frequency. The part of the system through which the signal passesA before any change in frequency occurs usually includes an antenna circuit and one or more resonant circuits with or without thermionic amplifying vacuum tubes and is generally termed the preselector.

The adjacent-channel selectivity obtainable in.

a superheterodyne system is largely due to the efficiency of the intermediate-frequency amplifier which, being operated at a fixed frequency,

may be designed to have a desired self-activity characteristic, The preselector of the receiving system serves to collect the desired signal, to aid in the rejection of undesired signals in adjacent channels, and to substantially reject signals of certain other frequencies to which a superheter- -odyne receiver tends to respond.

' quency of 1000 kilocycles, and if the intermediate frequency of the receiver is 460 kilocycles, the local oscillator will be adjusted to a frequency of 1460V kilocycles in order to convert the received signal to the intermediate frequency of 460 kilocycles, at which it is to be further amplified. If, at the same time, `a signal of 1920 -lrilocycles happens to be present at the input of the receiver, this signal, together with the oscillations of the local oscillator, will likewise produce a beat frequency of 460 kilocycles which will also be amplified by the intermediate frequency portion of the receiver, and will supply an undesirable response at the output thereof. This type of interference is commonly referred to as image response.

It is an object of the present invention to minimize or completely prevent such'image response in superheterodyne receivers in a simple, inexpensive and emcient manner. More specifically it is an object of the present invention to provide a preselector system for superheterodyne receivers and the like which affords effective suppression of image frequency response over the entire tuning range of the receiver.

My invention is intended for use in connection with resonant circuits which are tuned over a range of frequencies by nductance variation. Such a system is the one disclosed by Polydoroif in United States Patent No. Re. 21,282, in which a resonant circuit having an inductance coil and capacitor is adjusted over a range of frequencies by movement of a compressed comminuted ferromagnetic core relative to the inductance coil. This method of tuning is commonly called permeability tuning. An improved form of such a system is disclosed by Schaper in United States Patent No. 2,051,012. Both Polydoroffs original system and Schapers improved system may readily be ganged to provide multiple unit systems, Permeability tuning is especially adapted for use in the preselector of a superheterodyne radio receiver because its use permits the preselectcr to be designed so as to provide substantially uniform gain and selectivity throughout the frequency range, thus overcoming the chief cause of non-uniiorn'iity` of performance in this type of receiver.

-In accordance with the present invention, I achieve substantial elimination of response to the image-frequency signals by employing an arrangement in which such off-resonance signals produce opposed and substantially equal voltages in the input circuit of the first vacuum tube of the receiver, while the desired signal, to which the system is resonant, produces a normal resonant voltage in that circuit. To secure this result, I employ a varying mutual inductance across which one of the opposing imagefrequency voltages appears, and I arrange to automatically adjust the value of this mutual inductance, by the action of the tuning mea'ns,

y in such manner that the image-frequency voltized from the use of a single variably tuned resonant circuit in this portion of the receiver, thus ernster and aas-spve of the secondary and being A1d; inch long, and section'B being inch long. Both sections were woundwith No. 38 plain enamel wire, section A having 220 turnsper inch and sectionBhaving 24 turns per inch. The effective inductance of the primary i3 was 11.4 microhenries and the inductance of the secondary Il was 66.0 microhenries, both measured at 1560 kilocycles.

When the transformer just described was employed in a circuit arrangement as shown in Fig. l, capacitor 'l had a value of 300 micromicrofarads and capacitor! had a value of 315 paf' and was adjustable for purposes of alignment.`

Resistor 8 is included merely to provide a direct current grid r `l n path and' may suitably have :i value of in With core i inches long, and having relatlv-ely en ve permeability, resonant circuit #-l-t ce; f. tuned from 525 kllocycles 1560 ljlocycies without having te insert the core into the concentrated portion of primary i3. When aligned for maximum rejection by adjustment of capacitor 'l the system produced .image ratios up to 4350. By image ratio I mean the ratio of the input voltage of image frequency required to produce a given output, to the signal voltage required for the same output. In this embodiment the antenna gain varied from 2.3

to 5.8, while the mutual inductance varied from 13.35 microhenries to 53.85 microhenries.

The transformer constructionshown in Fig. 2 and described in detail above is very inexpensive to manufacture, as will be readily appreciated by those skilled in the production of such components. More elaborate development of the distribution of the primary winding,` and the use of more expensive conductors will produce designs having even better performance. I have chosen to show and describe this extremely simple form, however, to illustrate how, in accordance lwith the inventionexce1lent image'rejection can be `achieved with a minimum of .additional components and circuit complication.

Since image rejection is the principal function of the preselector circuit in modern superheterodyne receivers, and since the emphasis in the industry today is upon lo'w cost, my invention has wide and immediate application in broadcast receivers.

Having thus described my invention, what I claim is:

1. Arrangement for the suppression of undesired responses in ticularly in superheterodyne receivers, including a signal collector circuit, a'resonant circuit-tuned to a desired frequency, a mutual induct'ance between said collector circuit and said resonant capacitance connected mutual'inductance with respect to lvoltages induced from said collector circuit to said resonant circuit, and means for tuning said resonant circuit over a range of frequencies and for causing said mutual lnductance and said capacitance to have equal reactances at frequencies diifering by a substantially constant frequency interval from the frequencies to which said resonant circuit may be tuned.

3. `Arrangement for the suppression of undesired responses in radio receivers and thelike, particularly in superheterodyne receivers, including a signal collector circuit, a resonant circuit tuned to a desired frequency, a mutual induct ance between said collector circuit and said resonant circuit, a capacitance common to both said .circuits and in opposing series relation to said mutual inductance with respect to voltages induced from said collector circuit to said resonant circuit, and a movable lferromagnetic core for tuning said resonant circuit over a range of desired frequencies and for causing said mutual inductance to have equal reactances to said capacitance at frequencies differing by a substantially constant frequency interval from the frequenciesto which said resonant circuit'may be tuned.

4. An arrangement forthe suppression of undesired responses in radio receiversv and the like, particularly in superheterodyne receivers, including a transformer comprising lririinary and secondary windings arranged in'` opposed relation and having a common lowpotential terminal, a

ground, a resonant circui including said secondary and said capacitance and a 'ferromagnetic core movable relatively to d windings to tune said circuit to any desired of frequencies, said primary being so 'constructed and disposed relatively't said secondary that at'any position of said colle lthe mutual reactance of said'transformer is substantially equal to the reactance of said capacitance at frequencies difterval from the frequencies of said resonant cir-V radio reoeiversandthelike,par

circuit,and a capacitance common to both said circuits and in opposing series relation `to said mutual inductance with. respect to voltages induced from said collector-,circultto said resonant circuit, -said mutual inductance and said capacitancebeing so proportioned that at a frequency dliferent from the frequency to which said resonant circuit'is timed their reactances are substantially equal to one another. 1

. 2. Arrangement for the suppression. of unde- Y siredrosponsesinradioreeeivers'andthelike,v

particularly in superheterodyne receivers, includferlng by a substantially constant frequency incuit. l

5. A preselector system for radio receivers and the like, particularly `for superheterodynereceivers, including a transformer comprising coaxially disposed primary and secondary windingsarranged in opposed relation and having a common terminal, said primary. being disposed arolmd said secondary, a capacitor connected between said terminal and ground, a resonant circuit including said secondary and said capacitor and a ferromagnetic c'ore movable relatively to 'otsaidsecondarybuthasamajorporticnofiis ingasignalcollectorcircuita resonant circuit Y tunedtoadesiredfrequcncy,.amutualinduct ancebetweensaidcollectorcircuitandsaidresonantcircuihacapecitancecommontoboth circuit'and inopposingseriel relstionto mrnsconcentrated attheend opposite that at whichsaidcoremtemsaidcorebeingof'mch permeabilityastocoverthetuning rangerelquired forsaid resonant circuit without enterinsunt portion oftheswonusrywmcnisincident with the concentrated portion 'ot sold Ween/said terminal and signal within a range 'hapreseiectorsystemiorsupe r radioreceiversancithselikchavingsignalcoliectingmeansandanrstvacuumtubewithan input electrode, including a transformer having coaxially disposed primary .and secondary windingsarrangedinopposedrolationandhavinga` common terminal, said primary being connected to said signal collecting means and around said secondary winding, a capacitor connected between said terminal and ground, a resonant circuit including said secondary and said capacitorv vand connected between said input electrode and ground, and a ferromagnetic core movable relatively to said windings to tune said circuit to anydesired Bilhllwithin a rangeof irequencies. said primary and secondary' being so related relative to one another and to said core opposite that at which said core enters, said core being of such permeability as to cover the timing range required for -said resonant circuit without entering that portion of said secondary winding which is coincident with the concentrated portion of saidpi'mary.

' CK N. JACOB. 

